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Bi-induced p-type conductivity in nominally undoped Ga(AsBi)G. Pettinari, A. Patanè, A. Polimeni, M. Capizzi, Xianfeng Lu, and T. Tiedje Citation: Applied Physics Letters 100, 092109 (2012); doi: 10.1063/1.3690901 View online: http://dx.doi.org/10.1063/1.3690901 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Increased p-type conductivity in GaN x Sb1− x , experimental and theoretical aspects J. Appl. Phys. 118, 085708 (2015); 10.1063/1.4929751 Increased p-type conductivity through use of an indium surfactant in the growth of Mg-doped GaN Appl. Phys. Lett. 106, 222103 (2015); 10.1063/1.4922216 Electrical and optical properties of p-type InGaN Appl. Phys. Lett. 95, 261904 (2009); 10.1063/1.3279149 Electrical isolation of p-type GaAs layers by ion irradiation J. Appl. Phys. 91, 6585 (2002); 10.1063/1.1469693 Hole activation from GaAs:Zn nanoclusters for p-type conduction in ZnSe Appl. Phys. Lett. 76, 1701 (2000); 10.1063/1.126141

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Bi-induced p-type conductivity in nominally undoped Ga(AsBi)

G. Pettinari,1,a) A. Patanè,1 A. Polimeni,2 M. Capizzi,2 Xianfeng Lu,3,b) and T. Tiedje31School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom2CNISM-Dipartimento di Fisica, Sapienza Università di Roma, P.le A. Moro 2, 00185 Roma, Italy3Department of Electrical and Computer Engineering, University of Victoria, Victoria,British Columbia V8W 3P6, Canada

(Received 30 January 2012; accepted 12 February 2012; published online 1 March 2012)

We report p-type conductivity in nominally undoped GaAs1–xBix epilayers for a wide range ofBi-concentrations (0.6%� x� 10.6%). The counterintuitive increase of the conductivity withincreasing x is paralleled by an increase in the density of free holes by more than three orders ofmagnitude in the investigated Bi-concentration range. The p-type conductivity results from holesthermally excited from Bi-induced acceptor levels lying at 26.8 meV above the valence band edge

of GaAs1�xBix with concentration up to 2.4� 1017 cm�3 at x¼ 10.6%. VC 2012 American Instituteof Physics. [http://dx.doi.org/10.1063/1.3690901]

The unique electronic properties and uncommon de-

pendence on composition of the band structure in dilute bis-

mide alloys are attracting increasing interest and are

boosting several exciting lines of research in Materials Sci-

ence.1 Indeed, the large size of Bi-atoms leads to significant

relativistic corrections2,3 and a strong perturbation of the

GaAs host band structure parameters, such as the band-gap

energy4,5 and carrier effective mass.6,7 These properties

make dilute bismides of technological interest for several

applications that span from spintronics to high-efficiency so-

lar cells,8 heterojunction bipolar transistors,9 terahertz,10 and

infrared devices.11–13

Despite numerous studies on dilute bismides, the trans-

port properties of this material system are still largely

unknown and debated. Previous studies of GaAs1–xBix(x� 2.5%) have shown that the Bi incorporation does not de-grade significantly the electron transport properties of GaAs

(at least for x� 1.4%).14,15 This finding is in line with theintroduction by Bi-atoms of energy levels resonant in, or

close to, the GaAs valence band,16 thus affecting mainly the

hole transport. The first experimental studies of the effects of

Bi on the hole mobility (lh) have been reported only recentlyby Hall measurements on p-type (C- or Be-doped) GaAs1�xBix samples (x� 1.2%, see Ref. 17; and x� 5.5%, seeRef. 18) and by picosecond transient-grating technique

experiments on undoped GaAs1�xBix (2.5%� x� 6.3%, seeRef. 19). These works have revealed a general decrease of

the hole mobility following Bi incorporation. On the other

hand, the behavior of lh at high x (�10%) as well as theinfluence of Bi on the free-hole concentration (p) is still notclear. Kini et al.17 reported a large decrease of p withincreasing x (about one order of magnitude for x�1%) in C-and Be-doped GaAs1�xBix and ascribed this decrease to the

effect of BiGa heteroantisite defects, which act as double

donors and compensate partially the extrinsic p-typedoping of the alloy. In contrast, Nargelas et al.19 reportedp-type conductivity in undoped GaAs1�xBix samples

(2.5%� x� 6.3%), thus suggesting an acceptor behavior forBi in GaAs, but without reporting any compositional depend-

ence of p or lh.In this letter, we report p-type conductivity in nominally

undoped GaAs1�xBix epilayers for a wide range of Bi-concentrations (0.6%� x� 10.6%). Hall effect measure-ments reveal a monotonic increase of p with increasing x,which is accompanied by an increase of the conductivity.

These results support the existence of Bi-induced acceptor

levels,3,19 whose density increases with x. In the same con-centration range, the free-hole mobility exhibits an overall

decrease, with values in good quantitative agreement with

those in the literature.

We investigate a series of GaAs1�xBix epilayers

(x¼ 0.6%, 3.8%, 5.6%, 8.5%, and 10.6%; thicknesst¼ 30–56 nm) grown by solid source molecular beam epi-taxy on a semi-insulating (100) GaAs substrate. Further

details of the growth conditions and sample parameters can

be found elsewhere.5,7 The Bi-concentration was determined

by combining x-ray diffraction and optical data. All samples

were processed into 1-3-3-1 Hall bars of width W¼ 45 lmand length L¼ 1200 lm with Ti/Au metal depositionsproviding ohmic contacts [see inset in Fig. 2(b)]. Hall effect

measurements were performed using a high impedance

system with high current stability (�50 pA) in asuperconductive-magnet cryostat with magnetic field (B) upto 14 T and temperatures (T) ranging from 65 K to 290 K.

FIG. 1. Compositional dependence of the longitudinal conductivity r meas-ured in nominally undoped GaAs1�xBix epilayers at different temperatures.

a)Electronic mail: [email protected])Present address: Varian Semiconductor Equipment Associates, Gloucester,

Massachusetts 01930, USA.

0003-6951/2012/100(9)/092109/4/$30.00 VC 2012 American Institute of Physics100, 092109-1

APPLIED PHYSICS LETTERS 100, 092109 (2012)


19:10:11

http://dx.doi.org/10.1063/1.3690901http://dx.doi.org/10.1063/1.3690901

Figure 1 shows the longitudinal conductivity (r) atB¼ 0 T for GaAs1�xBix epilayers with different x. The datareveal a significant increase of r with increasing x, whichbecomes steeper with decreasing T. The inhomogeneity inthe conductivity along the Hall bar was measured to be less

than 20% in all samples. The Bi-induced increase of r differsfrom the systematic reduction of conductivity for increasing

alloying reported for other highly mismatched alloys, such

as Ga(AsN) and In(AsN).20,21 We investigate further this

counterintuitive behavior by Hall effect measurements.

For each sample, the Hall voltage (VH) was acquired intwo different experimental conditions: either as a function of

the longitudinal current (I) at fixed B (14 T) or as a functionof B at fixed I. In both configurations and in all samples, VHexhibits a linear dependence on I and B, see Fig. 2. The signof VH is consistent with a dominant p-type conductivity. Thevalues of the free-hole concentration p¼ (IB)/(eVHt) (wheree is the electron charge and t is the thickness of the GaAs1�xBix epilayers) are shown in Fig. 3(a) as a function of x atT¼ 250 K. Within the experimental uncertainty,22 the samevalues of p are obtained by measuring the Hall voltage atfixed or variable magnetic field. The measured free-hole con-

centration increases monotonically with increasing x, reach-ing values of �1017 cm�3 at x¼ 10.6%. This indicates theexistence of Bi-induced acceptors and supports previous

findings by different techniques; see Refs. 3 and 19. How-

ever, such an acceptor behavior of Bi-atoms is incompatible

with the presence of a large density of donor levels due to

BiGa heteroantisites suggested for p-type doped GaAs1�xBix

with x� 1.2%.17 Therefore, Bi-related donor states, if pres-ent, should become rapidly negligible with increasing x.

The Hall mobility of free holes, lh¼r/(ep), is shown inFig. 3(b) as a function of x at T¼ 250 K. Our results (dotsand circles) agree well with those reported in the literature

(stars), thus providing a general overview of the composi-

tional dependence of lh over a wide range of x.23 Although

the different sets of data show some scatter, a clear-cut

decrease of lh is observed for x up to �8.5%; whilst lhincreases from 0.9 6 0.1 cm2/(Vs) to 3.7 6 0.8 cm2/(Vs) forx going from 8.5% to 10.6%. This compositional dependenceof lh agrees with recent magneto-optical studies on the sameseries of samples.7 These measurements highlight a strong

hybridization between the Bi-related levels and the host

band-states for x< 6%. In turns, this results into an increaseof the carriers effective mass, paralleled here by a decrease

of the free-hole mobility. On the contrary, the softening of

this hybridization for x> 8% leads to a decrease of the car-riers effective mass, paralleled here by an increase of the

free-hole mobility.

A deeper insight into the mechanism responsible for the p-type conductivity in GaAs1�xBix was obtained from a detailed

temperature-dependent study of lh and p at x¼ 10.6%, as

FIG. 2. (Color online) (a) Hall voltage VH acquired at fixed magnetic field(B¼ 14 T) as a function of the longitudinal current I for different values ofthe Bi-concentration. (b) VH/I as a function of B for different x (I¼ 0.3 nA,2.5 nA, 60 nA, 35 nA, and 400 nA for x¼ 0.6%, 3.8%, 5.6%, 8.5%, and10.6%, respectively). Inset: optical-microscope image of a 1-3-3-1 Hall bar

employed for transport measurements. The dashed lines indicate, in both

panels, the slope for the linear Hall effect from free holes. FIG. 3. (Color online) (a) Compositional dependence of the free-hole con-

centration p in GaAs1�xBix at T¼ 250 K, as obtained from the analysis ofVH acquired for fixed (dots) and variable (circles) B. Inset: comparisonbetween experimental data of p and concentration of Bi-clusters of 1, 2, and3 Bi-atoms as estimated for a random alloy by including next-nearest-neigh-

bor interactions (solid lines), after Ref. 24. (b) As panel (a) for the free-hole

mobility, lh. Data of lh available in the literature are reported by stars(Refs. 17–19, at T¼ 300 K). The dashed line is a guide to the eye.

092109-2 Pettinari et al. Appl. Phys. Lett. 100, 092109 (2012)


19:10:11

reported in Fig. 4. The free-hole mobility increases monotoni-

cally with increasing T, as shown in Fig. 4(a), thus indicatingthat the contribution of elastic collisions by alloy disorder

and/or defects is significantly larger than that due to inelastic

collisions with phonons, at least for x¼ 10.6%. The depend-ence of p on temperature is shown in Fig. 4(b). It is welldescribed by the exponential law p¼ p0� exp(�Eb/kBT),where Eb¼ 26.8 6 0.5 meV is a single activation energy andp0¼ (2.4 6 0.1)� 1017 cm�3 is the density of acceptor levels.The value of Eb agrees with the binding energy of Bi-inducedacceptor levels (�25 meV) determined by far-infrared absorp-tion spectroscopy,3 whereas the value of p0 points towards asignificant concentration of acceptors. This gives strong evi-

dence that the p-type conductivity in GaAs1�xBix results fromholes thermally excited into the valence band from Bi-induced

acceptor levels. The intrinsic carrier contribution to the con-

ductivity is more than 5 orders of magnitude smaller than the

measured values and can be neglected even at room tempera-

ture, see dotted line in Fig. 4(b).

In the inset of Fig. 3(a), the compositional dependence

of p is compared with that of the density of clusters com-posed by 1, 2, or 3 Bi-atoms, as calculated for a random

distribution of impurities.24 This comparison indicates that

low-order Bi-clusters cannot account for the compositional

evolution of the acceptor density. The fast, exponential-like

increase of p with x suggests the existence of a driving forceruling the formation of the acceptor centers during the

growth process that cannot be explained in a pure random

framework. Theoretical investigations are now required to

address the geometry and number of Bi-atoms forming these

clusters or defect levels.

In conclusion, we have shown that the p-type conductiv-ity of nominally undoped GaAs1�xBix epilayers is enhancedby Bi atoms. This behavior differs fundamentally from that

found in other highly mismatched alloys and suggests that the

Bi-incorporation in GaAs leads to the formation of acceptor

levels lying at �27 meV above the valence band edge withconcentration of up to �2� 1017 cm�3 at x¼ 10.6%. The sys-tematic increase of the free-hole concentration with increasing

Bi-concentration is paralleled by a non-monotonic decrease of

the free-hole mobility, the latter being explained in terms of

the hybridization of the Bi-induced localized states with the

extended band-states of GaAs. These findings provide evi-

dence for an unique compositional dependence of the hole

transport properties of GaAs1�xBix that will stimulate further

theoretical works on the electronic properties of dilute bis-

mides. The Bi-induced increase of conductivity is also of

technological interest in the context of current research on the

device applications of dilute bismides.

This work was supported by the EU under grant agree-

ment No. PIEF-GA-2010-272612 and by the COST Action

MP0805. We are grateful to O. Makarovsky and D. Taylor

for their kind assistance during the transport measurements

and Hall bar processing.

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FIG. 4. (Color online) (a) Temperature-dependence (65 K�T� 290 K) ofthe free-hole mobility lh in GaAs1�xBix with x¼ 10.6%, as obtained fromthe analysis of VH acquired for fixed (dots) and variable (circles) B. Thedashed line represents the T3/2-dependence for impurity scattering. (b) Aspanel (a) for the free-hole concentration, p. An analysis in terms of freeholes thermally activated from a single acceptor level reproduces well the

experimental data for Eb¼ 26.8 meV and p0¼ 2.4� 1017 cm�3 (see solidline). The contribution of the intrinsic carrier concentration ni is largely neg-ligible (see dotted line).



19:10:11

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